Once having adopted firm convictions about the dangers of dietary fat based on his own limited research among small populations around the world, Keys repeatedly preached against the temptation to adopt any firm contrary convictions based on the many other studies of small populations that seemed to repudiate his hypothesis. “The data scarcely warrant any firm conclusion,” he would write about such contradictory evidence. When a 1964 article in JAMA, The Journal of the American Medical Association, for instance, reported that the mostly Italian population of Roseto, Pennsylvania, ate copious animal fat—eating prosciutto with an inch-thick rim of fat, and cooking with lard instead of olive oil—and yet had a “strikingly low” number of deaths from heart disease, Keys said it warranted “few conclusions and certainly cannot be accepted as evidence that calories and fats in the diet are not important.”

The Framingham Heart Study was an ideal example of this kind of selective thinking at work. The study was launched in 1950 under Thomas Dawber’s leadership to observe in a single community aspects of diet and lifestyle that might predispose its members to heart disease—risk factors of heart disease, as they would come to be called. The factory town of Framingham, Massachusetts, was chosen because it was what Dawber called a “reasonably typical” New England town. By 1952, fifty-one hundred Framingham residents had been recruited and subjected to comprehensive physicals, including, of course, cholesterol measurements. They were then re-examined every two years to see who got heart disease and who didn’t. High blood pressure, abnormal electrocardiograms, obesity, cigarette smoking, and genes (having close family with heart disease) were identified as factors that increased the risk of heart disease. In October 1961, Dawber announced that cholesterol was another one. The risk of heart disease for those Framingham men whose cholesterol had initially been over 260 mg/dl was five times greater than it was for men whose cholesterol had been under 200. This is considered one of the seminal discoveries in heart-disease research. It was touted as compelling evidence that Keys’s hypothesis was correct.

But there were caveats. As the men aged, those who succumbed to heart disease were ever more likely to have low cholesterol (as had Eisenhower) rather than high cholesterol. The cholesterol/heart-disease association was tenuous for women under fifty, and nonexistent for women older. Cholesterol has “no predictive value,” the Framingham investigators noted in 1971. This means women over fifty would have no reason to avoid fatty foods, because lowering their cholesterol by doing so would not lower their risk of heart disease. None of this was deemed relevant to the question of whether Keys’s hypothesis was true.